This invention relates to a retroreflector which may be used wherever light reflection is desired. A leading application of the retroreflector is as a retroreflective element of a roadmarker to provide directional guidance, and therefore it is described with respect to this use.
Roadmarkers are mounted on the surface of a roadway, such as along its center line or shoulders, to delineate paths or lanes for traffic, or at intersections to define stopping lines or cross-lanes for traffic, both vehicular and pedestrian. Markers of this type are mounted in spaced apart relation and serve to guide traffic in following or traversing a roadway, or in following a curve or grade in the roadway. Particularly to assist a driver of a vehicle at night, these markers have light reflectors which catch and return incident beams of light from vehicle headlights back toward the source of the light. Since automobiles of recent vintage have quite powerful headlights, the use of roadmarkers has become more widespread. Roadmarkers contribute to traffic safety such as when roads are wet from rain. Under certain conditions, such as fog, roadmarkers can be the only means of orienting a driver to a changing direction of a road.
Many forms of light reflectors have been suggested. They usually suffer from one or more limitations, such as reflecting too small a proportion of incident light while an approaching vehicle is still at an appreciable distance. As a result, reflecting markers are often noticed too late by a driver to be of substantial help.
Further, in order to avoid making a roadmarker an obstruction on the road, the marker preferably is designed to protrude only a slight amount from the road. This requirement augments problems of light reflection. Plain ceramic or plastic markers have been used, but they tend only to scatter the light. Light scattering is self-defeating in that it is accompanied by loss of intensity of the reflected light which materially reduces the effectiveness of the marker.
An effective reflecting system is a well known triple mirror reflex reflecting principle, such as is disclosed in U.S. Pat. No. 1,671,086 to Stimson, and which is referred to in the art as a cube-corner structure. While a cube-corner structure provides satisfactory performance as to light striking perpendicularly against an array or strip of cube-corners, that is, generally parallel to the axes of the cube-corners, this performance falls off fairly rapidly as incident light enters at angles away from the normal to the surface of the cube-corner array.